1. Field of the Invention
The present invention relates to a solid-state image pickup device in which a microlens is disposed immediately in front of a pixel opening which defines the shape of the opening of each constituent pixel element of a solid-state image pickup device.
2. Description of the Prior Art
In recent years, with the demand for higher quality images, a still image has become constituted of an increased number of pixels. For this reason, with respect to solid-state image pickup elements forming a light receiving section of a still video camera, a movie vide camera, and an electrical vide endoscope, the size of each pixel constituent element tends to inevitably become smaller. A pixel opening which defines the shape of the opening of each pixel constituent element (that is, the amount of light which enters each pixel constituent element) is formed only in a part of the over the entire surface of each pixel constituent element, and hence the amount of light to be detected is likely to become insufficient.
Conventionally, to prevent this, a microlens having an aperture larger than the pixel opening is disposed immediately in front of each pixel opening. The shortage of light entering each pixel constituent element is compensated by light-collecting action of this microlens, thereby preventing a drop in detection sensitivity.
For example, Unexamined Japanese Patent Publn. No. Hei-5(1993)-203837 discloses a solid-state image pickup device as shown in FIG. 1. A group of on-chip microlenses 18 consisting of a plurality of microlenses 20 is disposed on a chip-like solid-state image pickup element 16 which comprises a plurality of pixel constituent elements 12 arranged in a predetermined pattern, in such a way that each microlens 20 is positioned just above an each pixel opening 14. Luminous flux designated by the solid line arrow in the drawing entering the microlens 20 along the optical axis thereof is converged on the area of the pixel opening 14 by virtue of the effect of a convex lens of the microlens 20, thereby resulting in a solid-state image pickup element 16 of improved light converging efficiency and sensitivity.
However, the following problems stem from the previously mentioned solid-state image pickup device.
Specifically, if an image taking lens system disposed in front of the solid-state image pickup device has a sufficiently distant exit pupil, no particularly significant problem will arise. In contrast with this, if an image taking lens system having a close exit pupil is disposed in front of the solid-state image pickup device, light will enter the microlens 20 in an oblique direction with respect to the optical axis of the microlens 20 as the pixel constituent element moves away from the optical axis of the image taking lens system, as designated by a broken line. As a result of this, the point at which the light converges deviates from the pixel opening 14, and hence it becomes possible for the pixel opening 14 to acquire only a part of the luminous flux entering the microlens 20. For this reason, an image formed in the solid-state image pickup element 16 has a dark shade along the periphery of the image.